Our transcription: Although the Earth's magnetic field is similar to that of a bar magnet, there is one important difference. The strength of the Earth's field varies considerably over time. In fact, the Earth's field has decreased in strength about five percent over the last 150 years.Rocks can also have a magnetic field, but their magnetism is permanent. Iron rich rocks that have been strongly heated become magnetized by the Earth's magnetic field. When the rock cools, that magnetism is locked in becoming a permanent part of the rock. Because it's permanent, magnetized rocks of different ages are actually records of the Earth's magnetism throughout geologic time.